1. Field of the Invention
The present invention relates to a laser power controller for an apparatus for recording and reproducing information on an optical record medium such as an optical disk, an optical card or the like with a laser such as a laser diode. The present invention also relates to a control method of laser power.
2. Description of the Prior Art
In general, data are recorded and reproduced on an optical record medium such as an optical disk in the unit of area called as sector defined along a record track. A sector consists of three portions, an identification (ID) section, a gap section provided for controlling the laser power and a record area for recording information, and the three are arranged successively, as shown at the top of FIG. 3. Data are recorded only in record areas. The laser power has to be controlled optimally for recording and for reproduction, respectively.
On recording, the optical intensity of the laser beam on an optical disk has to be adjusted to have prescribed values in order to prevent errors. Because the optical intensity may change reflection property of an optical record medium erroneously, power control is carried out except the record areas, usually at a gap section located just before a record area. In an optical record medium, a gap section is arranged before a record area along the record track. Before recording, the laser beam irradiating the gap sections is monitored to determine appropriate drive conditions of the laser. Then, on recording, the laser is driven in the determined conditions to irradiate record areas according to record signals to change the reflection property at dot sites in the record areas, while on reproduction, the laser beam irradiates record areas to read information according to the light reflected from the record areas.
It is necessary to maintain the intensity of laser beam on the record medium at prescribed values on recording. If a laser diode is used as the laser, the characteristic of drive current against light-emitting power changes largely with ambient temperature and secular change of laser diode. FIGS. 1(a) and 1(b) show examples of the drive current vs. light-emitting power characteristic of a laser diode, wherein threshold value I.sub.th denotes a current at which the laser diode starts to emit light. As shown in FIG. 1(a), the threshold value I.sub.th increases and the slope coefficient .eta. or a slope of a curve of the drive current vs. light-emitting power characteristic decreases with increasing temperature. On the other hand, as shown in FIG. 1(b), the threshold value I.sub.th increases and the slope coefficient .eta. decreases as the laser diode operates for a long time. Therefore, it is needed to control a laser diode so as to keep the drive current at prescribed values under various conditions. Therefore, in an apparatus for recording and reproducing information optically, the laser is controlled so as to keep the optical intensity of laser beam on an optical record medium at prescribed values.
FIG. 2 shows an example of a block diagram of a prior art laser power controller, as disclosed in U.S. Pat. No. 4,845,720, filed Oct. 18, 1988, and issued to Koshi et al. A part of laser beam emitted from a laser diode 101 is detected with a photodetector 102 provided for monitoring. The photodetector 102 converts the light intensity of the incident laser beam to an electric signal, and an operational processor 103 determines the drive current of the laser diode 101 with reference to the output signal of the photodetector 102 so that the light-emitting intensity of the laser diode 101 keeps prescribed values. The operational processor 103 sets the result in a sample-and-hold circuit 114 and a driver 105 drives the laser diode 101 according to the output signal of the sample-and-hold circuit 114. A drive controller 110 sends signals 115, 116 for setting bottom and peak powers and a change signal 108 for changing between recording and reproduction, to the operational processor 103. These signals are used to change laser powers for recording. Further, the driver 105 drives the laser diode 101 according to the output signal received from the operational processor 103 and modulates the laser beam according to record signals 109 received from the drive controller 110. Thus, the record data are recorded correctly in an optical record medium.
FIG. 3 shows an example of the setting of record powers. The laser power control is performed in a gap section located just before a record area to be recorded, in order to determine two record powers, peak and bottom powers. A signal 115 for setting bottom power changes from "L" to "H" level to start the bottom power setting when the laser beam of the laser diode 101 enters a gap section, and the bottom power is determined in a period designated as BP. Next, when a signal 116 for setting peak power changes from "L" to "H" level, the determined value of bottom power is held in the sample-and-hold circuit 114 in a period designated as BS, and the setting of the peak power is started. Then, the peak power is determined in a period designated as PP. When the laser beam enters from the gap section to the record area, the change signal 108 changes from "L" to "H" level, and the setting value of the peak power is held in the sample-and-hold circuit 114 in a period designated as PS. After the two laser powers are held in the sample-and-hold circuit 114, the driver 105 performs recording according to record signals 109 received from the drive controller 110 by modulating the laser beam between the bottom and peak powers. When the laser beam passes the record area, the change signal 108 and the signals 115 and 116 are changed from "H" to "L" level at the same time, the sample-and-hold circuit 114 stops holding the signal, and the control of the record power is started again.
Usually, it takes about six to ten .mu.sec for the laser beam to pass a gap section. Therefore, the bandwidth of laser diode power control has a gain cross point at a few hundred kHz or higher in order to complete the setting of the bottom and peak powers in a gap section. Then, the gain cross point is set at a few hundred kHz to hasten the response of the setting of record powers. In this case, the bandwidth of read power is also extended in the same way, but there is no problem on the control of read power. In a gap section, the laser power is changed from the read power to the bottom power. The above-mentioned determination of the bottom power completes in three to four .mu.sec because the frequency band is widened by extending to higher frequencies. After completing the setting of the bottom power, it is sampled and held. Next, the peak power is set. The determination of peak power also completes in three to four .mu.sec, and the bottom power is sampled and held. After the determination of the record powers, the laser beam enters a record area, and the recording is carried out by modulating the laser beam between the determined bottom and peak powers.
As shown in FIGS. 1(a) and 1(b), the gain of the control loop changes according to the ambient temperature and the operating time. This changes the cross point of gain, so that the phase characteristic is also affected around the cross point. Then, the phase compensation is performed to give a margin for such a gain change in order to change the read power to the record power stably in a gap section. However, prior art laser power controllers have problems if the recording is carried out at a faster rate and at a higher density. If a time wherein the beam passes a gap section is shortened to for example a third of a time needed in prior art controllers, the bandwidth for the power control has to be widened to be extended to higher frequencies by three times, and the gain cross point of the control loop has to be increased to for example about one MHz. Then, it is needed to compensate the phase to have a margin even if the gain changes according to the temperature or the secular change of the laser diode. However, because the photodetector has a cut-off frequency at five to ten MHz, this cannot be neglected for the frequency characteristic having a gain cross point of about one MHz. Further, because the gain is changed to correct the scattering of slope efficiency and the like, the change of frequency characteristic due to the gain change has to be taken into account. If these factors are considered to increase processing rate, the circuit of the laser power controller becomes complicated. That is, the circuit scale becomes larger, more adjustment points have to be provided in the circuit, and the components used for the laser diode and for the photodetector have to be selected to satisfy prescribed specifications. Thus, the number of the components of the controller increases, and the cost thereof will increase.
If the cut-off frequency of the photodetector can be increased to about twenty MHz, the effect of the photodetector on the control characteristic can be neglected. However, this needs a photodetector and components of better quality, and this increases the cost thereof.